U.S. Pat. No. 4,083,412 discloses an ovoid rubber spring to transmit soil penetrating forces to the earth-working members of a seed drill. British Patent specification No. 1,274,219 discloses conical rubber springs for this purpose. In both prior art constructions the forces which resist increased deflection of the rubber springs in the working range due to encountering field obstructions is not constant and in fact increases abruptly. Consequently, as the soil-working members of the machine pass over rocks and the like the frame of the machine and such members must withstand much higher stresses than are encountered in normal operation. Because of this in the prior art, the soil-working members and frame must be designed to withstand these peak stresses, and as a result, are overdesigned for the normal operating loads. Another problem is that high shock loads are imposed on the openers as they are returned to operating positions after having passed over obstructions. This frequently results in field failures of coulters and openers.
The primary object of the present invention is to improve on the known prior art and to fully eliminate the above stated problems through the provision of a rubber spring for a seed drill which will deflect as required in response to the soil-working members of the machine passing or jumping over field obstructions without causing an increase in the force being applied to the spring to resist such deflections. The resistive force in the principle operating range provides for increasing pressures. If the soil-working members encounter an obstruction, the spring force increases to a desired maximum and then remains constant or decreases slightly. Therefore, the soil-working members and the frame of the seed drill do not feel significantly higher stresses than those normally encountered in field operation when the members encounter rocks or other obstructions. Because of this, shock loads on coulters and openers is lessened as they return to normal operating positions in contrast to the prior art employing ovoid or conical rubber springs.
Other features and advantages of the invention will be apparent to those skilled in the art during the course of the following description.